1. Field of the Invention
Embodiments of the invention generally pertain to the field of signal measurement and, more particularly, to optical interferometric measurement methods and apparatus utilizing a diffraction less optical beam such as a Bessel beam.
2. Description of Related Art
Optical interferometers of various configurations, e.g., Michelson, Mach-Zehnder, Sagnac, Fabry-Perot and others have been known for many years. These devices are used to detect and measure the optical interference produced by interfering light beams. The resulting interference pattern can be analyzed to measure a parameter of a disturbance affecting the path length of a sensing beam component with respect to a reference beam component of the interferometer.
The phenomenon of optical interference depends on the phase relationship between the interfering beams of light. When coherent beams interfere, this phase relationship produces constructive or destructive interference resulting in light and dark fringes, which can be analyzed in various known ways. Waves that are incoherent, when combined, produce rapidly moving areas of constructive and destructive interference and therefore do not produce a visible interference pattern. Light can be temporally coherent and/or spatially coherent. The temporal coherence of light is related to the spectral bandwidth of the source of the light. Thus a truly monochromatic or single frequency light transmission (wave) would have an infinite coherence time and coherence length. Spatial coherence, on the other hand, is the ability of any one spatial position of the wavefront of light to interfere with any other spatial position of the wavefront. Lasers emit light that typically has a high degree of both temporal and spatial coherence. Because some lasers emit highly collimated light, or the laser beam can easily be collimated, substantially plane wavefronts can be generated, which themselves have a high degree of spatial coherence. Therefore, lasers are used as the principle type of light source for most interferometers. However, due to the phenomenon of diffraction, a perfect or infinitely collimated beam cannot exist. Diffiraction can be expresssed as the apparent bending and spreading of waves when they meet an obstruction. Diffraction also occurs, for example, when a group of light waves of a finite size propagates through a medium such as free-space, causing the ‘collimated’ laser beam to eventually spread out as it travels farther from the source of the light.
Furthermore, a collimated laser beam typically has a Gaussian cross sectional intensity profile. The amplitude of the electric field of a Gaussian beam can be expressed by the following equationE(x,y,z)=E0(ω0/ωz)[exp−i[(2πnz/λ)−n(z)]−r2[ω−2+(iπn/λR(z))]]  (1)where [2πnz/λ)−n(z)] represents a longitudinal phase term and (iπn/λR(z)) represents a radial or transverse phase term. In applications requiring the propagation of a coherent beam through free-space, and subsequent detection, atmospheric turbulence and diffraction effects, for example, cause random fluctuations in these phase terms, which affect the quality of the propagating beam. These phase fluctuations ultimately limit the performance of optically based detection systems. In the exemplary application of coherent free-space sensing, random phase fluctuations induced by the atmosphere severely threaten the performance of coherent detection systems, especially at low frequencies. Various approaches to mitigating these effects, including the use of array receivers, multiple wavelengths, larger beam diameter, phase conjugation, beam propagation height adjustment, and others, have inherent disadvantages of their own. They include high cost, increased noise sensitivity, significant technical development effort and others.
In view of the foregoing, the inventors have recognized the need for a free-space beam propagation measurement method and apparatus that overcome these known disadvantages and others. Accordingly, embodiments of the invention are directed to interferometers and measurement methods that address the recognized shortcomings of the current state of technology, and which provide further benefits and advantages as those persons skilled in the art will appreciate.